In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. In some inkjet printers, the printhead ejects ink directly onto the surface of media as the media passes the printhead. The media can be in the form of a continuous web or in the form of sheets. In continuous web printers, the media are pulled from a supply roll by actuator-driven rollers. As the web moves through the printer it passes around rollers to which tension is applied to keep the web taut as it passes through the printer to a take-up roll. In sheet printers, actuator-driven rollers are positioned against one another to form nips and these nips urge the sheets through the printer.
Image quality in an inkjet printer relies on the flatness of the media/substrate that receives the ink drops ejected by the inkjets in a printhead during the printing process. Currently, inkjet systems rely on mechanical devices to hold the media/substrate flat to ensure a uniform dot placement. In these systems, the printhead must rely on the paper handling capability of the mechanical system to maintain media/substrate flatness. In a continuous web printing system, movable rollers are operated to maintain tension in the web to help maintain flatness in the web as it passes the printheads in the printer. In cut sheet systems, vacuum platens or similar structures hold a sheet flat as the sheet passes one or more printheads. Despite these systems, the media/substrate presents, at least on occasion, an uneven surface profile to the printheads. For example, even if the substrate is held flat by tension or a vacuum, textured media presents uneven surfaces for printing. Unfortunately, these changes in the profile of the media/substrate from the side view lead to a sloped surface that affects dot to dot placement which is directly proportional to that slope.
For example, FIG. 5 shows a forty-two μm spacing between inkjets in a printhead and a media sheet as it passes the printhead. As shown in the figure, if the sheet is flat, the distance between two drops ejected by the two inkjets that fly straight to the media surface is the same as the separation between the two inkjets, namely, L1, which is forty two μm. If the media surface is curved, as shown in the figure, the distance is L2, which is L1/cos θ where θ is the angle of the slope in the media. This distance is larger than L1. Consequently, these drops are separated by a greater distance than drops ejected by the two inkjets where the media is kept relatively flat. This greater separation can be perceived by the human eye in some ink images and is known as banding. A printer that can compensate for sloped surfaces in media being printed would be beneficial.